The present invention relates to a stable recombinant cell clone that is stable for at least 40 generations in serum- and protein-free medium, a biomass obtained by multiplying the stable cell clone under serum- and protein-free culturing conditions, and a method of preparing recombinant proteins by means of the biomass. Furthermore, the invention relates to a method of recovering stable recombinant cell clones. Furthermore, the invention relates to the production of a recombinant protein in a serum- and protein-free synthetic minimum medium.
Another aspect of the invention is a serum- and protein-free medium for culturing cells expressing a recombinant protein.
The preparation of recombinant proteins, in particular of biomedical products, such as blood factors, is gaining in importance. To allow for an optimum growth of recombinant cells, serum is added to the medium in most instances. Because of the high costs of serum and for avoiding possible contamination's in the culturing medium by viral or molecular pathogens from the serum, a number of serum-free media have been developed which, in particular, should not contain any additives of bovine or human origin. In addition to the low risk of contaminating the prepared products with viral and molecular pathogens, the use of such media in the preparation process also allows for a simpler purification of the expressed proteins.
In most instances, recombinant cells are first cultured in serum-containing medium up to a high cell density, e.g. for a working cell bank, and subsequently they are re-adapted to serum-free medium during the production phase.
Miyaji et al., Cytotechnology, 3:133–140 (1990) selected serum-independent cell clones in serum-free medium which contained insulin and transferrin. However, the living cell number and the expression rate proved to decrease continuously after 16 days. By co-amplification with a labeling gene, Mayaji et al., Cytotechnology, 4:173–180 (1990) tried to improve the expression rate and the productivity of the recombinant, cells.
Yamaguchi et al., Biosci. Biotechnol. Biochem., 56:600–604 (1992) established serum-independent recombinant CHO sub-clones by culturing serum-dependent cells on microtiter plates as monolayer for 3 to 4 weeks in serum-free medium that contained human serum albumin, insulin and transferrin. Approximately 0.1% of the cells were serum-independent. Part of the subclones also grew in suspension culture in serum-free medium, yet the cells aggregated and formed lumps. The duplicating time of the cells amounted to 1.5 days. Yet there are no data either on the stability of the serum-independent clones obtained, nor on the long time cultivation of these clones under serum-free conditions.
Media which allow for the maintenance of the metabolic activity and for a growth of cells during the serum-free phase frequently contain additional substances, e.g. growth factors, such as insulin or transferrin, or adherence factors which substitute the serum components.
To avoid the addition of polypeptide factors, such as insulin or transferrin, and to allow for protein free culturing conditions, various techniques have been developed. Thus, specifically defined, complete protein-free media have been developed which allow for a cell-growth also under protein-free conditions.
WO 97/05240 describes the preparation of recombinant proteins under protein-free conditions, the cells co-expressing a growth factor in addition to the desired protein.
JP 2696001 describes the use of a protein-free medium for the production of factor VIII in CHO cells by adding a non-ionic surface-active agent or cyclodextrin to increase the productivity of the host cells. To increase the effectiveness of these additives, the addition of, e.g., butyrate and lithium is recommended. As indicated in the specification, the addition of pluronic F-68 results in a marked increase in cell numbers.
WO 96/26266 describes the culturing of cells in a medium which contains a glutamine-containing protein hydrolysate whose content of free amino acids is less than 15 k of the total weight of the protein, and whose peptides have a molecular weight of less than 44 kD. As the culturing medium for the cell cultures, a synthetic minimum medium is used as the basic medium to which, inter alia, fetal calf serum, gentamycin and mercaptoethanol are added in addition to protein hydrolysate. The use of this serum-containing medium for the recombinant production of blood factors has not been mentioned.
U.S. Pat. No. 5,393,668 describes special synthetic surfaces which allow for a growth of adherent cells under protein-free conditions.
To stimulate cell proliferation, CHO cells which overexpress human insulin have been multiplied on an artificial substrate to which insulin is covalently bound (Ito et al., PNAS USA, 93:3598–3601 (1996)).
EP 0 872 487 describes the preparation of recombinant factor VIII in protein-free medium containing recombinant insulin to which polyols are added. According to the specification, the addition of pluronic F-68 results in an increased factor VIII productivity of BHK cells, and the addition of iron ions yet enhances this rise in productivity.
Reiter et al., Cytotechnology, 9:247–253 (1992) describe the immobilization of r-CHO cells first grown in serum-containing medium at a high density on carriers, and subsequent perfusion of the immobilized cells in protein-free medium during the production phase, wherein a continuous liberation of protein into the cell culture supernatant was found. There, the cells were perfused for less than 10 generations in protein-free medium.
Katinger et al., Adv. In Mol. Cell Biol., 15a: 193–207 (1996) describe the preparation of stable cell cultures wherein the cells are immobilized on macroporous carriers. It is emphasized that perfusion cultures with porous carrier materials would be preferable to other methods. Stable clones expressing recombinant proteins, such as FVIII or von-Willebrand factor, are not described, the cells are invariably grown first in serum-containing medium and are only later transferred to serum- and protein-free medium.
Previous methods for the successful preparation of a large-scale cell culture under protein-free conditions have been described for continuous cell lines, in particular VERO cells (see, e.g., WO 96/15231). There, the cells are grown under serum- and protein-free conditions from the original ampule up to a large technical scale of 1200 1. However, these are not recombinant cells, but host cells which are used for the production of virus antigen in a lytic process.
In contrast to adherent VERO cells, e.g. CHO cells are dependent on adhesion to a limited extent only. CHO cells grown by means of conventional methods under serum-containing conditions are capable of binding both to smooth and to porous micro-carriers (U.S. Pat. No. 4,978,616; Reiter et al., Cytotechnology, 9:247–253 (1992)). If CHO cells are grown under serum-free conditions, they lose this property and do not adhere to smooth carriers, such as, e.g., Cytodex 3, or they detach easily therefrom, unless adherence-promoting additives, such as, e.g., fibronectin, are put into the medium. Because of the slight adherence of CHO cells to carriers under serum-free conditions, the production of recombinant proteins thus mainly is effected in suspension culture. There, the production process may be effected as a continuous or as a batch-wise method. The recombinant cell culture at first is grown in a bioreactor up to an optimum cell density, optionally the protein expression is induced, and for harvesting, the medium containing the expressed proteins but also recombinant cells is withdrawn at certain intervals from the reaction tank and thus from the production process. By the continuous loss of biomass, the production efficiency in the bioreactor drops and increases again slowly only after the addition of fresh medium, since the cells must grow up to the desired cell density. Thus, despite the continuous process, repeatedly there is a phase of retardation, in which the production rate in this system drops. Furthermore, the growth and production capacity in such a system is limited by the maximum cell density attainable.
When adapting cells initially grown under serum containing conditions to protein-free medium, it has repeatedly been found that the yield of expressed protein and the productivity of recombinant CHO cells greatly drops after adaptation in protein-free medium as compared to serum-containing conditions (Paterson et al., Appl. Microbiol. Biotechnol., 40:691–658 (1994)). This is the consequence of an instability or reduced growth of the recombinant clones due to the changed culturing conditions. Despite the use of a stable original clone, on account of the altered fermentation conditions, repeatedly a large portion of the cells become cells with reduced expression or also non-producers, which overgrow product producers during the production process, whereby the fermented culture finally largely consists of non-producers or of such cells having a low expression.
As a consequence, the maximum production capacity of the fermentation culture drops continuously, and a maximum product production is restricted to a certain number of generations or cell passages.
Thus, there is a need for a system in which a continuous production is possible over as long a period of time as possible, in particular in the large-scale production of recombinant proteins under serum- and protein-free conditions.
It would furthermore be desirable to obtain a recombinant cell clone which is stable in the production phase for many generations under protein free conditions and which expresses recombinant protein.